Consolidated Balance Sheets (unaudited) (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	60,638,672	60,149,411
Common stock, shares outstanding	57,701,124	57,788,573
Treasury stock, shares	2,937,548	2,360,838